It's not that your kata was "hated", no. It's simply that Codewars collected a lot of content through they years which has many quality issues and caused troubles to users in one or another way, and requirements for kata and authors became quite strict, to prevent this. Put reviewers with poor social skills on top of this and you get what you experienced.

Two main reasons for the negative feedback your kata collected could be:

• It's similar to other kata. Other than Vigenere kata I pointed you to, there's a variety of Caesar cipher kata, etc. You can see the extent of the problem here. Community does not like duplicates, but it's still not well defined what is a duplicate. Some reviewers have stricter opinion on the topic than others.
• It was published in a shape not acceptable to many users. Problem with beta kata is that they sometimes can get approved prematurely while still being in bad shape. It should not happen, but some users just love clicking whatever button they see, and "Approve kata" button is one of them. To partially prevent this problem, community worked out a method of retiring bad beta kata quickly. Recently, to countermeasure this phenomenon and support beginner authors, a set of help articles was published:
  • Authoring guidelines which your kata should meet to be considered good for approval. There are still some placeholders there, but it's mostly finished and stable.
  • Creating your first kata tutorial, which I do not like too much personally, but might be helpful. It's not published yet and unfortunately I have no time to finish it.
  • Tutorial on Creating kata in Python, and there are also versions for some other languages (JS, C, other ones incoming when I find time).

Docs are still work in progress in general, there's a lot of TODOs and gaps to fill, but they slowly get better and better. If you are going to create more kata, be sure to read through the linked articles, and if you do not understand something, ask. Articles explain how to get feedback early without risk of being bashed by reviewers with social skills of an ogre.

Since the kata got retired, you do not have to do anything more. You cannot delete it.

Your encryption patter seems like a case of Vigenère cipher, and such kata already exists.

so please don't take my words out of context again

Huh? I don't see where that's the case.

I just thought this would be a fun first kata for me to make

And that's fine, but I'm afraid it won't leave beta phase, as it's already at the edge of retirement. No one really left any feedback for you why they downvoted it, but I'd guess it's similar reasons to what I'm saying.

would be different from any out there

In what way? Have you actually checked for similar katas? Just from reading description I'm pretty sure this exact kata exists already, if not 95+% similar. And even if there weren't, adding 1 if statement to it wouldn't make it unique.

But this is clearly a duplicate of many other caesar cipher katas, no?

You can read this documentation on creating a kata.